Varactor diodes, also known as varicap diodes, are a type of semiconductor diode used for a wide range of frequency control applications. The Varactor diode is characterized by its use as a voltage controlled capacitor. In essence, it works similarly to a normal diode, except that when a reverse bias voltage is applied to the diode, the capacitance or junction capacitance is affected by the voltage. As the reverse voltage increases, the capacitance decreases. This phenomenon is used to tune the resonant frequency of circuits or to modify the frequency response of amplifiers. Varactor diodes are used in oscillators and filters in various communication circuits and in test equipment.